Low temperature in-situ combustion



Oct. 5, 1965 J. D. ALEXANDER ETAL 3,209,825

LOW TEMPERATURE IN-SITU COMBUSTION Filed Feb. 14, 1962 INVENTORS D.ALEXANDER, N. DEwe AM l.. MARTIN D/57J4NCE FROM INJECTION POI/V7'ATToRNEYS United States Patent O 3,209,825 LOW TEMPERATURE IN-SITUCOMBUSTION John D. Alexander, John N. Dew, and William L. Martin,

Ponca City, Okla., assignors to Continental Oil Colnpany, Ponca City,Okla., a corporation of Delaware Filed Feb. 14, 1962, Ser. No. 173,24914 Claims. (Cl. 16d- 11) This invention relates, as indicated, to animproved in-situ` combustion process. More particularly, the presentinvention relates to a method of recovering oil from a subsurfaceformation by a low temperature in-situ combustion process.

As it is well known in the producing division of the oil industry, asubstantial amount of time and effort is being expended in perfectingthe recovery of oil from partially depleted reservoirs by processesinvolving burning of a portion of the hydrocarbons in the reservoir.Several in-situ combustion processes have been proposed and tried.Results to date indicate that this type of process has substantial meritif certain difficulties can be overcome.

In substantially all previous in-situ combustion processes, an injectionwell is surrounded by a plurality of recovery wells, with all of thewells traversing the formation to be treated. The initial step in theprocess is to force air or some other gas through the formation from theinjection well to the recovery wells to deter-mine that gas can beforced through the formation. The second step in the process is theignition of at least a portion of the hydrocarbons in the formationaround the injection well to form a flame front which is sustained byinjection of air through the injection well, such that the flame frontproceeds radially from the injection well toward the recovery wells. Oneof the major problems in the process is the ignition of hydrocarbons inthe formation in an economical manner without damage to the equipment inthe injection well.

Perhaps the most popular method of ignition is locating a suitableheater in the injection well bore adjacent the formation to be treated,such that the air injected into the formation from the injection well isheated by the heater to a temperature sufficient to ignite thehydrocarbons on the face of the formation `formed at the injection well.This method has two serious objections. First, the high temperaturesreached in the injection well bore (sometimes as high as 1200 degreesF.) seriously damage the well casing and cement and the heater has ashort service life at these temperatures. Secondly, the rather suddenheating of the formation at the injection Well frequently causespermeability blocking which is primarily caused by vaporization of waterin the formation and subsequent cooling or condensation of the water inthe colder formation ahead of the flame front. When permeabilityblocking occurs, air cannot be forced through the4 formation to sustainthe flame front.

Another ignition method involves preheating an inert gas (such asnitrogen) at the surface and then injecting the heated inert gas downthrough the Well bore into the formation to heat the formation to anignition temperature; whereupon air is substituted for the inert gas toignite hydrocarbons in the formation and form a flame front. This methodis also disadvantageous since inert gas is expensive and' not readilyobtainable; large heat losses occur in forcing the heated gas throughthe length of the injection well bore, and excessive temperatures arecreated in the well -bore with results similar to those described above.As a variation of this method, it has also been proposed to let theinjection well cool off to, say 400 degrees F., from an initialtemperature of from 650 F. to 900 degrees F., prior to injection of air.in order 3,299,825 Patented Oct. 5, 1965 to provide ignition in theformation at some distance from the injection well. This is undoubtedlyan improvement since it reduces the maximum temperatures attained in theinjection well, but the high temperature of the inert gas injected inthe formation provides permeability blocking and a coking of the oil inthe formation around the injection well which requires an appreciabletemperature for the ignition of the coke. Also, of course, the high heatlosses in forcing the heated inert gas through the length of theinjection well remain.

The present invention contemplates a novel process of in-situ:combustion utilizing low temperature oxidation of 'oil in a formationaround an injection well. As a preliminary step, a `gas (such as air) isforced through the formation from the injection well to one or morereco-very Wells to establish that gas can be forced through theformation. The air being injected is .then lslowly heated in theinjection well to a temperature greater than the vaporizationtemperature of the -formation water, but less than the cokingtemperature of the oil in the formation. As a result, permeabilityblocking is prevented and residual oil will remain in the formationaround the injection well. Continued injection of the heated air slowlyoxidizes the residual oil around the injection well, and the ignitiontemperature of the oil is reached at some distance from the injectionWell. It will thus be apparent that the llame front is formed away fromthe injection well t-o prevent excessive temperatures in the injectionwell .which would cause damage to equipment in the injection well. Theflame front is directed on through the formation by continued injectionof .air through the injection Well. As used herein, the term dame frontmeans the high temperature combustion front which occurs after theignition temperature of the oil is reached.

More specifically, the present invention contemplates a method ofrecovering oil from a subsurface formation containing oil and water and.traversed by an injection well yand at least 4one recovery Well,comprising the steps of:

(a) forcing air through the formation from the injection well to therecovery well,

(b) slowly increasing the temperature of the air to a temperaturesufficiently high to oxidize oil in the formation, but to a temperatureless than a coking temperature of the oil,

(c) moving the flame front provided Iby said oxidation toward therecovery well, and

(d) recovering the gases yand liquids flowing from the formation 4intothe recovery well.

An important object of this invention is to eihciently and economicallyrecover oil from a .subsurface formation.

Another object of this invention is to recover oil from a subsurfaceformation by in-:situ combustion without damage to any of the equipmentinstalled in the injection well.

Another object of this `invention is to minimize the possibility ofpermeability blocking in an in-'situ combustion process.

A further object of this invention is to provide a maximum `service lifefor la heater used Iin an injection Wellin an in-situ combustionprocess.

A further object of this invention is to initially ignite hydrocarbonsin a formation subjected .to in-situ :combustion at a minimumtemperature and in spaced relation from the injection well.

A still further object of this invention is to provide an in-situcombustion process having minimum energy requirements.

Other objects and advantages of the invention will be evident from thefollowing detailed description, when read in conjunction with theaccompanying drawings which illustrate this invention.

In the drawings:

FIG. 1 is a plan view of a typical injection and recovery well patternused in an in-situ combustion process.

FIG. 2 is a vertical sectional view through the injection well and oneof the recovery wells, as taken along lines 22 of FIG. 1.

FIG. 3 is a set of typical heat curves obtained in a process accordingto this invention.

Referring to the drawings in detail, and particularly FIG. l, referencecharacter designates an injection well and reference character 12designates each of a plurality of recovery wells arranged incircumferentially spaced relation around the injection well. In atypical arrangement, tive of the recovery wells 12 are drilled aroundthe injection well 10 to provide adequate drainage points for uidsforced radially outward from the injection well 10, as will be explainedbelow.

As shown in FIG. 2, the injection well 10 and each of the recovery wells12 are drilled to such a depth as to traverse the formation 14 to betreated. It will be understood, of course, that the wells 10 and 12 mayhave all been producing wells utilized in recovering oil from theformation 14 by artificial lifting means, such as pumps, until the flowfrom the formation 14 was reduced to such an extent that the use ofpumps became uneconomical. On the other hand, a portion of the wells 10and 12 may be drilled specifically for utilization of the in-situcombustion process of this invention.

The injection well 10 may be completed open hole, but is normallyprovided with a suitable casing 16 cemented in place and havingperforations 18 therein extending through the zone of the injection welltraversing the formation 14. The recovery wells 12 may be completed inany desired fashion, as long as the iluids from the formation 14 mayeasily enter the recovery wells and be removed from the recovery wells.The recovery well 12 shown in FIG. 2 is shown as being completed openhole and no equipment is shown therein, to simplify the illustration.

In accordance with the present invention, suitable packers 20 and 22 areplaced in the injection well 10 at the upper and lower ends of theformation 14 to isolate the Zone 24 of the injection well whichtraverses the formation 14. It will be understood by those skilled inthe art that the lower packer 22 may be eliminated if the injection wellterminates approximately at the bottom of the formation 14, and if noperforations are provided below the formation 14. A suitable tubingstring 26 is extended downwardly through the injection well 10, throughthe upper packer 20 and into the upper portion of the zone 24. Asuitable heater 28 is supported on the lower end of the tubing string 26within the upper portion of the zone 24. The heater 28 may be of anydesired construction, such as electric or gas-tired, and is soconstructed that gas forced downwardly through the tubing string 26 willbe heated by the heater 28 before owing into the zone 24.

With the heat equipment installed in the injection well 10 asillustrated in FIG. 2, air or any other suitable gas is forced throughthe tubing string 26 and through the heater 28 into the zone 24. The gasflows from the zone 24 through the perforations 18 and into and throughthe formation 14 toward the recovery wells 12. The pressure required toforce the gas through the formation 14 will depend upon thecharacteristics of the formation 14, but is typically about 600 p.s.i.When the gas begins discharging into the recovery wells 12, showing thatgas can be pumped through the formation 14, the heater 28 is placed inoperation to start heating the gas being forced through the formation14.

The heater 28 is tired in such a manner as to slowly or gradually heatthe gas being forced through the formation 14, and hence to slowly orgradually heat the formation. We have found it particularly useful tooperate the heater 28 in such a manner as to heat the gas being injectedin a stepwise or incremental fashion. For example, in one eld test ofthe present process, the gas (air) was heated and forced through theformation at 200 degrees F. for five days; then 300 degrees F. for threedays; then 400 degrees F. for two days; then 600 degrees F. for fivedays. With this gradual heating, no permeability blocking wasencountered. It may also be noted that at each step, from 200,000 to800,000 cubic feet of air per day were forced through the formationbeing treated.

In a preferred embodiment of this invention, the gas forced or pumpedthrough the formation 14 in all stages of the process is air, since thisis the most economical gas to use and, as shown, has highly usefulresults. It is to be understood, however, that the gas used fordetermining that gas can be forced through the formation 14 and forinitially heating the formation 14 may be an inert gas, such asnitrogen, if desired. In this latter event, oxygen must be supplied tothe formation 14 through the injection well 10 to oxidize hydrocarbonsin the formation and initiate combustion, as is described below, whenthe necessary temperature of the formation is reached.

The temperature curves shown in FIG. 3 were obtained from a laboratorytest conducted in accordance with the process set forth herein, and arebelieved to be representative of the increase in temperature of any oilbearing formation treated in accordance with our invention. It will beobserved in FIG. 3 that the temperature of the air being injected wasgradually raised from about 300 degrees F. to approximately 475 degreesF. and retained at this latter temperature. This test data clearly showsthat the particular oil being used began to oxidize at a temperature ofapproximately 425 degrees. As the temperature of the injection airincreased on up to 450 and 475 degrees, oxidation of the oil increasedin such a manner that the temperature of the core sample downstream ofthe injection point began to increase and continued to increase untilthe ignition temperature of approximately 875 degrees F. was reached.Thus, when air is injected into a formation from an injection well andis slowly heated as described above, the oil in the formation 14 aroundthe injection well 10 will begin to oxidize, yet ignition will takeplace radially outward from the injection well 10 to protect theinjection Well 10 from excessive temperatures.

In the past, it has been the practice to heat a formation around aninjection well to temperatures substantially above the cokingtemperature of the oil in the formation (from 550 F. to 700 F.). Underthese conditions, the coke at the face of the walls of the injectionwell or the coke immediately adjacent the injection well was ignited.This procedure is desirable in the sense that all of the valuablehydrocarbons should be recovered and only the low value coke would beburned. We have found, however, that coke has a high ignitiontemperature, cornpared with residual oil. In our process, the formationaround the injection well 10 is gradually heated to drive off thelighter hydrocarbons, but the temperature of the formation adjacent theinjection well is not raised into the coking temperature range of theoil until a ame front has been formed away from the well bore. As aresult, the initial ignition of the hydrocarbons in the formation 14around the injection well is established with much lower well boretemperatures than have been used in the past. It will be understood, ofcourse, that once ignition is made and the flame front is established,the temperature at the flame front will be above the coking temperatureof the oil and will drive otf all of the hydrocarbons except for thecoke, such that only coke will be burned in the bulk of reservoir space.An added advantage of slowly heating the gas injected in the formation14 is that the formation water is gradually vaporized and forced throughthe formation 14 without forming a permeability block as is frequentlyencountered with prior processes, as indicated above.

The injection of air through the injection well is continued afterestablishment of the flame front in order to lsustain the ame front anddrive the flame front radially toward the recovery wells 12. Also, it ispreferred that the heater 28 be continued in operation after ignition isestablished to continually preheat the air being fed to the formation.This sustains the flame front to prevent the possibility of excessiveheat loss behind the ame front and extinguishment of the iiame.

The gases and liquids driven from the formation 14 into the recoverywells 12 may be recovered in any desired manner which will preserve thevaluable hydrocarbons. The recovery of these hydrocarbons from the wells12 is not, in itself, a part of our invention, and may be performed bywell known procedures.

From the foregoing it will be apparent that the present inventionprovides `a convenient and economical process of recovering oil from asubsurface formation. The temperature of the injection Well is retainedsufficiently low as not to damage any of the equipment in the injectionwell, and the heater used in the injection well will have a long servicelife. The flame front is formed radially outward from the injection welland is formed without permeability blocking. It will also be apparentthat the present process will have a minimum of energy requirements.

Changes may be made in the combination and arrangement of steps orprocedures as heretofore set forth in this specification and shown inthe drawings, it being understood that changes may be made withoutdeparting from the spirit and scope of the invention as defined in thefollowing claims.

VWhat is claimed is:

1. A method of recovering oil from a subsurface formation containing oiland water and traversed by an injection well and at least one recoveryWell, comprising the steps of:

(a) forcing gas through the formation from the injecjection Well to therecovery well;

(b) Slowly, over a period of days, increasing the temperature of saidgas within the injection well and while the resulting heated gas isbeing injected into the formation from the injection well to atemperature greater than the vaporization temperature of the formationWater, but less than the coking temperature of the oil in the formation,to retain residual oil in the formation around the injection well;

(c) injecting air into the formation from the injection well at atemperature sufficiently high to oxidize the residual oil for initiatingcombustion of the residual oil and forming a flame front around theinjection well;

(d) continuing the injection of air into the formation from theinjection well until the iiame front formed by combustion of the oil inthe formation has advanced into proximity with the recovery well, and

(e) recovering the gases and liquids flowing into the recovery Well fromthe formation.

2. A method of recovering oil from a subsurface formation containing oiland water and traversed by an injection well and at least one recoverywell, comprising the steps of (a) forcing air through the formation fromthe injection well to the recovery well;

(b) slowly over a period of days, increasing the temperature of said airWithin the injection well and while the resulting heated air is beinginjected into the formation from the injection Well to a temperatureless than the coking temperature of the oil in the formation, butsufficiently high to oxidize residual oil in the formation around theinjection well to form a flame front in the formation;

(c) continuing the injection of air into the formation through theinjection well for moving the fiame front 6 formed by said oxidationtoward the recovery well, and

(d) recovering the gases and liquids owing into the recovery Well fromthe formation.

3. The method defined in claim 2 wherein the temperature of the airinjected through the injection well is increased stepwise.

4. The method defined in claim 2 wherein said air is heated within theinjection well in the zone of the well traversing the formation.

5. The method defined in claim 2 wherein the air injected into theformation through the injection well for moving the flame front towardthe recovery well is heated to a temperature sutiiciently high tooxidize residual oil in the formation.

6. The method defined in claim 2 wherein the temperature of the air isbetween about 300 F. and about 700 F.

7. The method defined in claim 6 wherein the temperature of said air instep (a) is greater than the vaporization temperature of the formationwater.

8. The method defined in claim 2 wherein the temperature of the air isbetween about 300 F. and about 550 F.

`9. The method defined in claim 2 wherein the temperature of the air isbetween about 300 F. and about 475 F.

10. A method of recovering oil from a subsurface formation containingoil and Water, comprising the steps of:

(a) providing an injection Well from the surface through the formation;

(b) providing a plurality of recovery Wells in circumferentially spacedrelation around the injection well, with each -of said recovery wellsbeing drilled through the formation;

(c) isolating the zone of the injection well traversing the formation;

(d) forcing gas through the formation from the injection well to therecovery wells,

(e) placing a heater in said zone,

(f) activating said heater,

(g) pumping air through said heater and said zone into the formation,

(h) controlling the temperature of said heater to slowly increase thetemperature of the air being forced into the formation from said zone,over a period of days, until the temperature of the air exceeds theVaporization temperature of the water in the formation but is less thanthe coking temperature of the oil in the formation, for forcingformation Water and lighter hydrocarbons through the formation towardthe recovery well and initiating combustion of residual oil in theformation .around the injection well, and

(i) recovering the gases and liquids flowing from the formation into therecovery wells.

11. A method of recovering oil from a subsurface formation containingoil and water and traversed by an injection well and at least onerecovery Well, comprising the `steps of:

(a) gradually, over a period of days, heating the formation from theinjection well towards the recovevry well with a gas heated within theinjection well and While the resulting heated gas is being injected intothe formation from the injection well to a temperature less than thecoking temperature of the oil to drive the water and lighter hydrocarboncomponents of the oil toward the recovery well;

(b) igniting the residual oil in the formation around the injection wellwith air heated to a temperature less than the coking temperature of theoil in the formation to form a flame front;

(c) injecting air into the formation from the inject-ion well to drivethe flame front toward the recovery well; and

(d) recovering the liquid and gaseous.` hydrocarbons from the recoverywell.

"i 8 12. The method defined in claim 11 wherein the tern- (c) continuinginjecting air into the formation from perature of said gas is betweenabout 300 F. and about the injection weil to drive the flame fronttoward the 700 F. recovery well; and

13. A method of recovering oil from a subsurface (d) recovering liquidand gaseous hydrocarbons from formation containing oil and water andtraversed by an linjection well and at least one recovery well,comprising the recovery well. 14. The method defined in claim 13 whereinthe ternthe steps of: perature of said air is between about 300 F. andabout (a) introducing heated air into the formation from the 700 F.

injection well; References Cited by the Examiner injection wellsimultaneously while the heated air is l. being injected into saidformation from the injection l5 BENJAMIN HERSH P'lmary Exa'mner well toform a aine front in the formation; BENJAMIN BENDETT, Examiner.

2. A METHOD OF RECOVERING OIL FROM A SUBSURFACE FORMATION CONTAINING OILAND WATER AND TRAVERSED BY AN INJECTION WELL AND AT LEAST ONE RECOVERYWELL, COMPRISING THE STEPS OF: (A) FORCING AIR THROUGH THE FORMATIONFROM THE INJECTION WELL TO THE RECOVERY WELL; (B) SLOWLY OVER A PERIODOF DAYS, INCREASING THE TEMPERATURE OF SAID AIR WITHIN THE INJECTIONWELL AND WHILE THE RESULTING HEATED AIR IS BEING INJECTED INTO THEFORMATION FROM THE INJECTION WELL TO A TEMPERATURE LESS THAN THE COKINGTEMPERATURE OF THE OIL IN THE FORMATION, BUT SUFFICIENTLY HIGH TOOXISIZE RESIDUAL OIL IN THE FORMATION AROUND THE INJECTION WELL TO FORMA FLAME FRONT IN THE FORMATION; (C) CONTINUING THE INJECTION OF AIR INTOTHE FORMATION THROUGH THE INJECTION WELL FOR MOVING THE FLAME FRONTFORMED BY SAID OXIDATION TOWARD THE RECOVERY WELL, AND (D) RECOVERINGTHE GASES AND LIQUIDS FLOWING INTO THE RECOVERY WELL FROM THE FORMATION.